求助 一个简单计算器的代码

初学c++时写的，还很不完善，没有用到很多c++的许多独有的特性，仅供参考一下，有括号sqrt()表示开根号
#include <iostream>
#include <string>
#include <map>
#include <math.h>

using namespace std;
map<string,double>table;
double expr(bool);
double term(bool);
double prim(bool);
double error(const string&);


enum Token_value{NAME,       NUMBER,    END,     SQR,   EXIT,
                 PLUS='+',   MINUS='-', MUL='*', DIV='/',
                 ASSIGN='=', PRINT=';', LP='(',  RP=')'};

Token_value curr_tok=PRINT;        //定义一个全局变量；
Token_value get_token();


double expr(bool get)              //加减运算
{
    double left=term(get);
    for(;;)
    {switch (curr_tok)
    {
    case PLUS:
        left+=term(true);
        break;
    case MINUS:
        left-=term(true);
        break;
    default:
        return left;    
    }
    }
}

double term(bool get)             //处理乘除运算
{
    double left=prim(get);
    for(;;)
    {switch (curr_tok)
    {
    case MUL:
        left*=prim(true);
        break;
    case DIV:
        if (double b=prim(true))
        {
         left/=b;
         break;
        }
        return error("divide by 0!");
    default:
        return left;
    }
    }
}


double number_value;
string string_value;

double prim(bool get)                 //处理关键字和括号
{if(get) get_token();
 switch (curr_tok)
 {
 case NUMBER:
     {double v=number_value;
      get_token();
      return v;
     }
 case NAME:
     { double& v=table[string_value];
       if(get_token()==ASSIGN) v=expr(true);
       return v;
     }
 case SQR:
      {double v=sqrt(prim(true));
       return v;
      }
 case MINUS:
     {
      return -prim(true);
     }
 case LP:
     {double e=expr(true);
      if(curr_tok!=RP) return error(") is expected!");
      get_token();
      return e;
     }
 default:
     return error("primary expected!");
 }
}

Token_value get_token()
{   char ch;
    do
    {if(!cin.get(ch)) return curr_tok=END;
    }
    while (ch!='\n'&&isspace(ch));
    switch (ch)
    {
    case ';':
    case '\n':
        return curr_tok=PRINT;
    case '+':
    case '-':
    case '*':
    case '/':
    case '=':
    case '(':
    case ')':
        return curr_tok=Token_value(ch);
    case '0':case '1':case '2':case '3':case '4':
    case '5':case '6':case '7':case '8':case '9':case '.':
        cin.putback(ch);
        cin>>number_value;
        return curr_tok=NUMBER;
    default:
        if(isalpha(ch))
        {
        string_value=ch;
        while (cin.get(ch)&&isalnum(ch))
        string_value.push_back(ch);
         cin.putback(ch);   
         if (string_value=="sqrt")
          {return curr_tok=SQR; }
          else if(string_value=="exit")
          {return curr_tok=EXIT;
          }                      //添加
         return curr_tok=NAME;
        
        }
      error ("bad token!");
      return curr_tok=PRINT;
    }
}
int no_error;
double error(const string& s)
{   no_error++;
    cerr<<"error:"<<s<<endl;
    return no_error;
}

int main()
{
    table["pi"]=3.14159;
    table["eq"]=2.718281;

    while (cin)
    {get_token();
    if(curr_tok==END) break;
    if(curr_tok==PRINT) continue;
    if(curr_tok==EXIT) break;
    cout<<expr(false)<<"\n";
    }
    system("pause");
    return no_error;
}

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